docs: def dup_idx(arr):
      """Return the indices of all duplicated array elements.
  
      Parameters
      ----------
      arr : array-like object
          An array-like object
  
      Returns
      -------
      idx : NumPy array
          An array containing the indices of the duplicated elements
  
      Examples
      --------
      >>> from root_numpy import dup_idx
      >>> dup_idx([1, 2, 3, 4, 5])
      array([], dtype=int64)
      >>> dup_idx([1, 2, 3, 4, 5, 5])
      array([4, 5])
      >>> dup_idx([1, 2, 3, 4, 5, 5, 1])
      array([0, 4, 5, 6])
  
      """
      _, b = np.unique(arr, return_inverse=True)
      return np.nonzero(np.logical_or.reduce(
          b[:, np.newaxis] == np.nonzero(np.bincount(b) > 1),
          axis=1))[0]